FIG. 1 is a schematic illustration (not to scale) showing an example of a conventional free space optical communication (FSO) receiver system 100 with fine tracking control.
This type of FSO receiver system is typically associated with a communications receiver on a mobile platform, for example, a ground vehicle, aircraft, or ship. This type of FSO receiver system is typically implemented to mitigate against shake and jitter generated by vehicle movement which can lead to signal drop-out.
In this example, the FSO receiver system 100 comprises a tip-tilt mirror 102, a beam splitter 104, a focussing lens 106, an optical receiver 108, a Fourier transform lens 110, a quad tracking sensor 112, and a tracking controller 114.
In operation, a light beam 116 is incident on the system 100. The light beam 116 is an optical communications signal specifying communications data. The received light beam 116 is incident on the tip-tilt mirror 102. The tip-tilt mirror 102 reflects the incident light beam 116 onto the beam splitter 104. The beam splitter 104 then splits the received light beam 116 into two separate beams, namely a first beam 118 and a second beam 120. In some examples, the second beam 120 may be a relatively small fraction (for example 10%) of the light beam 116, and the first beam 118 may be the remainder.
The first beam 118 is directed by the beam splitter 104 onto the focussing lens 106. The focussing lens 106 focuses the first beam 118 (thereby to produce the first focussed beam 122) onto the optical receiver 108. The optical receiver 108 measures the first focussed beam 122, thereby determining the communications data.
The second beam 120 is directed by the beam splitter 104 onto the Fourier transform lens 110. The Fourier transform lens 110 focuses the second beam 120 (thereby to produce the second focussed beam 124) onto the quad tracking sensor 112. The quad tracking sensor 112 measures the second focussed beam 124.
The quad tracking sensor 112 is described in more detail later below with reference to FIG. 4. In this example, the quad tracking sensor 112 is an angle-of-arrival sensor that is axially aligned with the optical receiver 108. In this example, the quad tracking sensor 112 comprises four optical sensors, hereinafter referred to as “quadrant sensors”. The quad tracking sensor 112 is coupled to the tracking controller 114 such that an output of the quad tracking sensor 112 is sent to the tracking controller 114.
The tracking controller 114 is configured to, using the received output of the quad tracking sensor 112, control the tip-tilt mirror 102. In particular, in this example, the tracking controller 114 implements a tracking control algorithm to periodically update the power of the portion of the second focussed beam 124 detected on each of the quadrant sensors of the quad tracking sensor 112. Using this information, the tracking controller 114 periodically determines the position of the point on the quad tracking sensor 112 to which the second focussed beam 124 is focussed, or the offset error of this point from a centre of the quad tracking sensor 112. The tracking controller 114 then uses this positional information to periodically determine a corrective angular movement for the tip-tilt mirror 102 that would cause the second focussed beam 124 to be focussed at the centre of the quad tracking sensor 112 (i.e. to cause the point on the quad tracking sensor 112 at which the second focussed beam 124 is focussed to move to the centre of the quad tracking sensor 112). This process may be referred to as centroiding.
The tracking controller 114 then periodically controls the tip-tilt mirror 102 as determined, thereby causing the second focussed beam 124 to be focussed at the centre of the quad tracking sensor 112. As the quad tracking sensor 112 is axially aligned with the optical receiver 108, this movement of the tip-tilt mirror 102 tends to cause the first focussed beam 122 to be focussed at a desired point (e.g. a centre) of the optical receiver 108.
Thus, the tip-tilt mirror 102 is periodically controlled such that the optical receiver 108 receives an optical signal (i.e. the first focussed beam 122) at a substantially optimum position, thereby reducing signal drop out.
FIG. 2 is a schematic illustration (not to scale) showing a top view of a portion of an alternative example conventional FSO receiver system with fine tracking control.
FIG. 3 is a schematic illustration (not to scale) showing a side view of the portion of the alternative example conventional FSO receiver system shown in FIG. 2.
In this example, the beam splitter (not shown in FIGS. 2 and 3) is configured to split a received light beam into the first and second light beams 118, 120 such that the first and second light beams 118, 120 are substantially parallel to each other.
In this example, the focussing lens 106, the optical receiver 108, the Fourier transform lens 110, and the quad tracking sensor 112 are fixed to a fixture 200.
The focussing lens 106 and the Fourier transform lens 110 are positioned aligned and side-by-side on an upper surface of the fixture 200. Similarly the optical receiver 108 and the quad tracking sensor 112 are positioned aligned and side-by-side on the upper surface of the fixture 200.
The focussing lens 106 and the optical receiver 108 are arranged on the fixture 200 such the focussing lens 106 focuses the first beam 118 onto the optical receiver 108. Similarly, the Fourier transform lens 110 and the quad tracking sensor 112 are arranged on the fixture 200 such the Fourier transform lens 110 focuses the second beam 120 onto the quad tracking sensor 112.
In a similar way to that performed in the first example described in more detail earlier above with reference to FIG. 1, in this example, the tracking controller (not shown in FIGS. 2 and 3) uses an output of the quad tracking sensor 112 to control pan and tilt movement of the fixture 200. This pan and tilt movement is illustrated in FIG. 3 by double-headed arrows and the reference numerals 300 and 302 respectively.
In particular, in this example, the tracking controller implements a tracking control algorithm to continuously or periodically update the power of the portion of the second focussed beam 124 detected on each of the quadrant sensors of the quad tracking sensor 112. Using this information, the tracking controller determines the position of the point on the quad tracking sensor 112 to which the second focussed beam 124 is focussed, or the offset error of this point from a centre of the quad tracking sensor 112. The tracking controller then uses this positional information to determine a corrective pan/tilt movement 300, 302 for the fixture 200 to cause the second focussed beam 124 to be focussed at the centre of the quad tracking sensor 112.
FIG. 4 is a schematic illustration (not to scale) showing certain details of a front surface of the quad tracking sensor 112.
In this example, the quad tracking sensor 112 comprises four optical sensors, namely a first quadrant sensor 401, a second quadrant sensor 402, a third quadrant sensor 403, and a fourth quadrant sensor 404.
In this example, the front surface of the quad tracking sensor 112 is substantially circular. Each quadrant sensor 401-404 forms a respective quarter of the circular front surface of the quad tracking sensor 112. In this example, the quadrant sensors 401-404 are substantially the same shape as each other. In this example, the quadrant sensors 401-404 are substantially the same size as each other.
Each quadrant sensor 401-404 is configured to measure the power of the portion of the second focussed beam 124 that is incident on it.
The tracking controller 114 is configured to determine the position of the point on the quad tracking sensor 112 onto which the second focussed beam 124 is focussed (and/or the offset error of this point from the centre of the quad tracking sensor 112) using the respective power measurements taken by the quadrant sensors 401-404.